Polypeptide growth factors play a key role in regulating the development of multicellular organisms and in the processes of tissue maintenance and repair. (For recent reviews see Cross and Dexter (1991) Cell, Vol. 64, pp. 271-280; Aaronson (1991) Science, Vol. 254, pp. 1146-1153.) At the cellular level, growth factors are involved in regulating proliferation and the progressive acquisition of the differentiated phenotype. Growth factors are capable of stimulating cellular proliferation as well as inhibiting cellular proliferation and many growth factors have been found to be multifunctional (Sporn and Roberts (1988) Nature, Vol. 332, pp. 217-219). The highly coordinated functions of growth factors is perhaps best exemplified in the development of the hematopoiectic cell system (Metcalf (1989) Nature, Vol. 339, pp. 27-30) where a limited number of stem cells give rise to a larger population of developmentally restricted progenitor cells. These progenitor cells are further stimulated to proliferate and differentiate into mature lymphoid, erythroid and myeloid cells.
Much of what we know about the functions of growth factors comes from in vitro studies using target cell lines that are capable of responding to growth factors in some measurable way. The ability of a particular target cell to respond to a growth factor is determined by the expression of a specific receptor which binds the factor and initiates a signal transduction cascade resulting in a cellular response. Furthermore, the nature of the response to a particular growth factor is often a function of the type of cell and the differentiated state of the cell. Thus, our ability to understand the function of known growth factors and to identify novel growth factors will be enhanced by expanding the repertoire of target cells representative of various cell lineages.